The biological role of lectins (non-enzymatic carbohydrate-binding proteins of non-immune origin; I. J. Goldstein et al., 1980, Nature 285:66) is inextricably linked to that of carbohydrates. One function of carbohydrates is the modification of physical characteristics of glyco-conjugates (i.e., solubility, stability, activity, susceptibility to enzyme or antibody recognition), however, a more interesting and relevant aspect of carbohydrate biology has emerged in recent years; the carbohydrate portions of glyco-conjugates are information rich molecules (N. Sharon and H. Lis, 1989, Science 246:227-234; K. Drickamer and M. Taylor, 1993, Annu. Rev. Cell Biol. 9:237-264; A. Varki, 1993, Glycobiol. 3:97-130). Within limits, the binding of carbohydrates by lectins is specific (i.e., there are lectins that bind only galactose or N-acetylgalactose; other lectins bind mannose; still others bind sialic acid and so on; K. Drickamer and M. Taylor, supra). Specificity of binding enables lectins to decode information contained in the carbohydrate portion of glyco-conjugates and thereby mediate many important biological functions.
Numerous mammalian, plant, microbial and viral lectins have been described (I. Ofek and N. Sharon, 1990, Current Topics in Microbiol. and Immunol. 151:91113; K. Drickamer and M. Taylor, supra; I. J. Goldstein and R. D. Poretz, 1986, in The Lectins, p.p. 33-247; A. Varki, supra). These proteins mediate a diverse array of biological processes which include: trafficking of lysosomal enzymes, clearance of serum proteins, endocytosis, phagocytosis, opsonization, microbial and viral infections, toxin binding, fertilization, immune and inflammatory responses, cell adhesion and migration in development and in pathological conditions such as metastasis. Roles in symbiosis and host defense have been proposed for plant lectins but remain controversial. While the functional role of some lectins is well understood, that of many others is understood poorly or not at all.
The diversity and importance of processes mediated by lectins is illustrated by two well documented mammalian lectins, the asialoglycoprotein receptor and the serum mannose binding protein, and by the viral lectin, influenza virus hemagglutinin. The hepatic asialoglycoprotein receptor specifically binds galactose and N-acetylgalactose and thereby mediates the clearance of serum glycoproteins that present terminal N-acetylgalactose or galactose residues, exposed by the prior removal of a terminal sialic acid. The human mannose-binding protein (MBP) is a serum protein that binds terminal mannose, fucose and N-acetylglucosamine residues. These terminal residues are common on microbes but not mammalian glyco-conjugates. The binding specificity of MBP constitutes a non-immune mechanism for distinguishing self from non-self and mediates host defense through opsonization and complement fixation. Influenza virus hemagglutinin mediates the initial step of infection, attachment to nasal epithelial cells, by binding sialic acid residues of cell-surface receptors.
The diversity of lectin mediated functions provides a vast array of potential therapeutic targets for lectin antagonists. Both lectins that bind endogenous carbohydrates and those that bind exogenous carbohydrates are target candidates. For example, antagonists to the mammalian selecting, a family of endogenous carbohydrate binding lectins, may have therapeutic applications in a variety of leukocyte-mediated disease states Inhibition of selectin binding to its receptor blocks cellular adhesion and consequently may be useful in treating inflammation, coagulation, transplant rejection, tumor metastasis, rheumatoid arthritis, reperfusion injury, stroke, myocardial infarction, burns, psoriasis, multiple sclerosis, bacterial sepsis, hypovolaemic and traumatic shock, acute lung injury, and ARDS.
The selecting, E-, P- and L-, are three homologous C-type lectins that recognize the tetrasaccharide, sialyl-Lewisx (C. Foxall et al, 1992, J. Cell Biol. 117,895-902). Selectins mediate the initial adhesion of neutrophils and monocytes to activated vascular endothelium at sites of inflammation (R. S. Cotran et al., 1986, J. Exp. Med. 164, 661-; M. A. Jutila et al., 1989, J. Immunol. 143,3318-; J. G. Geng et al., 1990, Nature, 757; U. H. Von Adrian et al., 1994, Am. J. Physiol. Heart Circ. Physiol. 263, H1034-H1044). In addition, L-selectin is responsible for the homing of lymphocytes to peripheral and mesenteric lymph nodes (W. M. Gallatin et al., 1983, Nature 304,30; T. K. Kishimoto et al., 1990, Proc. Natl. Acad. Sci. 87,2244-) and P-selectin mediates the adherence of platelets to neutrophils and monocytes (S-C. Hsu-Lin et al., 1984, J. Biol. Chem. 259,9121).
Selectin antagonists (antibodies and carbohydrates) have been shown to block the extravasation of neutrophils at sites of inflammation (P. Piscueta and F. W. Luscinskas, 1994, Am. J. Pathol. 145, 461-469), to be efficacious in animal models of ischemia/reperfusion (A. S. Weyrich et al., 1993, J. Clin. Invest. 91,2620-2629; R. K. Winn et al., 1993, J. Clin. Invest. 92, 2042-2047), acute lung injury (M. S. Mulligan et al., 1993, J. Immunol. 151, 6410-6417; A. Seekamp et al., 1994, Am. J. Pathol. 144, 592-598), insulitis/diabetes (X. D. Yang et al., 1993, Proc. Natl. Acad. Sci. 90,10494-10498), meningitis (C. Granet et al., 1994, J. Clin. Invest. 93, 929-936), hemorrhagic shock (R. K. Winn et al., 1994, Am J. Physiol. Heart Circ. Physiol. 267, H2391-H2397) and transplantation. In addition, selectin expression has been documented in models of arthritis (F. Jamar et al., 1995, Radiology 194, 843-850), experimental allergic encephalomyelitis (J. M. Dopp et al., 1994, J. Neuroimmunol. 54, 129-144), cutaneous inflammation (A. Siber et al., 1994, Lab. Invest. 70, 163-170) glomerulonephritis (P. G. Tipping et al., 1994, Kidney Int. 46, 79-88), on leukaemic cells and colon carcinomas (R. M. Lafrenie et al., 1994, Eur. J. Cancer [A] 30A, 2151-2158) and L-selectin receptors have been observed in myelinated regions of the central nervous system (K. Huang et al., 1991, J. Clin. Invest. 88, 1778-1783). These animal model data strongly support the expectation of a therapeutic role for selectin antagonists in a wide variety of disease states in which host tissue damage is neutrophil-mediated.
Other examples of lectins that recognize endogenous carbohydrates are CD22β, CD23, CD44 and sperm lectins (A. Varki, 1993, Glycobiol.3, 97-130; P. M. Wassarman, 1988, Ann. Rev. Biochem. 57, 415442). CD22β is involved in early stages of B lymphocyte activation; antagonists may modulate the immune response. CD23 is the low affinity IgE receptor; antagonists may modulate the IgE response in allergies and asthma. CD44 binds hyaluronic acid and thereby mediates cell/cell and cell/matrix adhesion; antagonists may modulate the inflammatory response. Sperm lectins are thought to be involved in sperm/egg adhesion and in the acrosomal response; antagonists may be effective contraceptives, either by blocking adhesion or by inducing a premature, spermicidal acrosomal response.
Antagonists to lectins that recognize exogenous carbohydrates may have wide application for the prevention of infectious diseases. Many viruses (influenza A, B and C; Sendhi, Newcastle disease, coronavirus, rotavirus, encephalomyelitis virus, enchephalomyocarditis virus, reovirus, paramyxovirus) use lectins on the surface of the viral particle for attachment to cells, a prerequisite for infection; antagonists to these lectins are expected to prevent infection (A. Varki, 1993, Glycobiol.3, 97-130). Similarly colonization/infection strategies of many bacteria utilize cell surface lectins to adhere to mammalian cell surface glyco-conjugates. Antagonists to bacterial cell surface lectins are expected to have therapeutic potential for a wide spectrum of bacterial infections, including: gastric (Helicobacter pylori), urinary tract (E. coli), pulmonary (Klebsiella pneumoniae, Stretococcus pneumoniae, Mycoplasma pneumoniae) and oral (Actinomyces naeslundi and Actinomyces viscosus) colonization/infection (S. N. Abraham, 1994, Bacterial Adhesins, in The Handbook of Immunuopharmacology: Adhesion Molecules, C. D. Wegner, ed; B. J. Mann et al., 1991, Proc. Natl. Acad. Sci. 88, 3248-3252). A specific bacterial mediated disease state is Pseudomonas aeruginosa infection, the leading cause of morbidity and mortality in cystic fibrosis patients. The expectation that high affinity antagonists will have efficacy in treating P. aeruginosa infection is based on three observations. First, a bacterial cell surface, GalNAcβ1-4Gal binding lectin mediates infection by adherence to asialogangliosides (αGM1 and αGM2) of pulmonary epithelium (L. Imundo et al., 1995, Proc. Natl. Acad. Sci 92, 3019-3023). Second, in vitro, the binding of P. aeruginosa is competed by the gangliosides' tetrasaccharide moiety, Galβ1-3GalNAcβ1-4Galβ1-4Glc. Third, in vivo, instillation of antibodies to Pseudomonas surface antigens can prevent lung and pleural damage (J. F. Pittet et al., 1993, J. Clin. Invest. 92, 1221-1228).
Non-bacterial microbes that utilize lectins to initiate infection include Entamoeba histalytica (a Gal specific lectin that mediates adhesion to intestinal mucosa; W. A. Petri, Jr., 1991, AMS News 57:299-306) and Plasmodium faciparum (a lectin specific for the terminal Neu5Ac(a2-3)Gal of glycophorin A of erthrocytes; P. A. Orlandi et al., 1992, J. Cell Biol. 116:901-909). Antagonists to these lectins are potential therapeutics for dysentery and malaria.
Toxins are another class of proteins that recognize exogenous carbohydrates (K-A Karlsson, 1989, Ann. Rev. Biochem. 58:309-350). Toxins are complex, two domain molecules, composed of a functional and a cell recognition/adhesion domain. The adhesion domain is often a lectin (i.e., bacterial toxins: pertussis toxin, cholera toxin, heat labile toxin, verotoxin and tetanus toxin; plant toxins: ricin and abrin). Lectin antagonists are expected to prevent these toxins from binding their target cells and consequently to be useful as antitoxins.
There are still other conditions for which the role of lectins is currently speculative. For example, genetic mutations result in reduced levels of the serum mannose-binding protein (MBP). Infants who have insufficient levels of this lectin suffer from severe infections, but adults do not. The high frequency of mutations in both oriental and Caucasian populations suggests a condition may exist in which low levels of serum mannose-binding protein are advantageous. Rheumatoid arthritis (RA) may be such a condition. The severity of RA is correlated with an increase in IgG antibodies lacking terminal galactose residues on Fc region carbohydrates (A. Young et al., 1991, Arth. Rheum. 34, 1425-1429; I. M. Roitt et al., 1988, J. Autoimm. 1, 499-506). Unlike their normal counterpart, these gal-deficient carbohydrates are substrates for MBP. MBP/IgG immunocomplexes may contribute to host tissue damage through complement activation. Similarly, the eosinophil basic protein is cytotoxic. If the cytotoxicity is mediated by the lectin activity of this protein, then a lectin antagonist may have therapeutic applications in treating eosinophil mediated lung damage.
Lectin antagonists may also be useful as imaging agents or diagnostics. For example, E-selectin antagonists may be used to image inflamed endothelium. Similarly antagonists to specific serum lectins, i.e. mannose-binding protein, may also be useful in quantitating protein levels.
Lectins are often complex, multi-domain, multimeric proteins. However, the carbohydrate-binding activity of mammalian lectins is normally the property of a carbohydrate recognition domain or CRD. The CRDs of mammalian lectins fall into three phylogenetically conserved classes: C-type, S-type and P-type (K. Drickamer and M. E. Taylor, 1993, Annu. Rev. Cell Biol. 9, 237-264). C-type lectins require Ca++ for ligand binding, are extracellular membrane and soluble proteins and, as a class, bind a variety of carbohydrates. S-type lectins are most active under reducing conditions, occur both intra- and extracellularly, bind β-galactosides and do not require Ca++. P-type lectins bind mannose 6-phosphate as their primary ligand.
Although lectin specificity is usually expressed in terms of monosaccharides and/or oligosacchrides (i.e., MBP binds mannose, fucose and N-acetylglucosamine), the affinity for monosaccharides is weak. The dissociation constants for monomeric saccharides are typically in the millimolar range (Y. C. Lee, 1992, FASEB J. 6:3193-3200; G. D. Glick et al., 1991, J Biol. Chem. 266:23660-23669; Y. Nagata and M. M. Burger, 1974, J. Biol. Chem. 249:116-3122).
Co-crystals of MBP complexed with mannose oligomers offer insight into the molecular limitations on affinity and specificity of C-type lectins (W. I. Weis et al., 1992, Nature 360:127-134; K. Drickamer, 1993, Biochem. Soc. Trans. 21:456-459). The 3- and 4-hydroxyl groups of mannose form coordination bonds with bound Ca++ ion #2 and hydrogen bonds with glutamic acid (185 and 193) and asparagine (187 and 206). The limited contacts between the CRD and bound sugar are consistent with its spectrum of monosaccharide binding; N-acetylglucosamine has equatorial 3- and 4-hydroxyls while fucose has similarly configured hydroxyls at the 2 and 3 positions.
The affinity of the mannose-binding protein and other lectins for their natural ligands is greater than that for monosaccharides. Increased specificity and affinity can be accomplished by establishing additional contacts between a protein and its ligand (K. Drickamer, 1993, supra) either by 1) additional contacts with the terminal sugar (i.e., chicken hepatic lectin binds N-acetylglucose amine with greater affinity than mannose or fucose suggesting interaction with the 2-substituent); 2) clustering of CRDs for binding complex oligosaccharides (i.e., the mammalian asialylglycoprotein receptor); 3) interactions with additional saccharide residues (i.e., the lectin domain of selectins appears to interact with two residues of the tetrasaccharide sialyl-Lewisx: with the charged terminal residue, sialic acid, and with the fucose residue; wheat germ agglutinin appears to interact with all three residues of trimers of N-acetylglucosamine); or by 4) contacts with a non-carbohydrate portion of a glyco-protein.
The low affinity of lectins for mono- and oligo-saccharides presents major difficulties in developing high affinity antagonists that may be useful therapeutics. Approaches that have been used to develop antagonists are similar to those that occur in nature: 1) addition or modification of substituents to increase the number of interactions; and 2) multimerization of simple ligands.
The first approach has had limited success. For example, homologues of sialic acid have been analyzed for affinity to influenza virus hemagglutinin (S. J. Watowich et al. 1994, Structure 2:719-731). The dissociation constants of the best analogues are 30 to 300 μM which is only 10 to 100-fold better than the standard monosaccharide.
Modifications of carbohydrate ligands to the selectins have also had limited success. In static ELISA competition assays, sialyl-Lewisa and sialyl-Lewisx have IC50s of 220 μM and 750 μM, respectively, for the inhibition of the binding of an E-selectin/IgG chimera to immobilized sialyl-Lewisx (R. M. Nelson et al., 1993, J. Clin. Invest. 91, 1157-1166). The IC50 of a sialyl-Lewisa derivative (addition of an aliphatic aglycone to the GlcNAc and replacement of the N-acetyl with an NH2 group) improved 10-fold to 21 μM. Similarly, removal of the N-acetyl from sialyl-lewisx improves inhibition in an assay dependent manner (C. Foxall et al., 1992, J. Cell Biol. 117, 895-902; S. A. DeFrees et al., 1993, J. Am. Chem. Soc. 115, 7549-7550).
The second approach, multimerization of simple ligands, can lead to dramatic improvements in affinity for lectins that bind complex carbohydrates (Y. C. Lee, supra). On the other hand, the approach does not show great enhancement for lectins that bind simple oligosaccharides; dimerizing sialyl-Lewisx, a minimal carbohydrate ligand for E-selectin, improves inhibition approximately 5-fold (S. A. DeFrees et al., supra).
An alternative approach is to design compounds that are chemically unrelated to the natural ligand. In the static ELISA competition assays inositol polyanions inhibit L- and P-selectin binding with IC50s that range from 1.4 μM to 2.8 mM (O. Cecconi et al., 1994, J. Biol. Chem. 269, 15060-15066). Synthetic oligopeptides, based on selectin amino acid sequences, inhibit neutrophil binding to immobilized P-selectin with IC50s ranging from 50 μM to 1 mM (J-G Geng et al., 1992, J of Biol. Chem. 267, 19846-19853).
Lectins are nearly ideal targets for isolation of antagonists by SELEX technology described below. The reason is that oligonucleotide ligands that are bound to the carbohydrate binding site can be specifically eluted with the relevant sugar(s). Oligonucleotide ligands with affinities that are several orders of magnitude greater than that of the competing sugar can be obtained by the appropriate manipulation of the nucleic acid ligand to competitor ratio. Since the carbohydrate binding site is the active site of a lectin, essentially all ligands isolated by this procedure will be antagonists. In addition, these SELEX ligands will exhibit much greater specificity than monomeric and oligomeric saccharides.
A method for the in vitro evolution of nucleic acid molecules with highly specific binding to target molecules has been developed. This method, Systematic Evolution of Ligands by EXponential enrichment, termed SELEX, is described in U.S. patent application Ser. No. 07/536,428, entitled “Systematic Evolution of Ligands by Exponential Enrichment,” now abandoned, U.S. patent application Ser. No. 07/714,131, filed Jun. 10, 1991, entitled “Nucleic Acid Ligands,” now U.S. Pat. No. 5,475,096, U.S. patent application Ser. No. 07/931,473, filed Aug. 17, 1992, entitled “Methods for Identifying Nucleic Acid Ligands,” now U.S. Pat. No. 5,270,163 (see also PCT/US91/04078), each of which is herein specifically incorporated by reference. Each of these applications, collectively referred to herein as the SELEX Patent Applications, describes a fundamentally novel method for making a nucleic acid ligand to any desired target molecule.
The SELEX method involves selection from a mixture of candidate oligonucleotides and step-wise iterations of binding, partitioning and amplification, using the same general selection scheme, to achieve virtually any desired criterion of binding affinity and selectivity. Starting from a mixture of nucleic acids, preferably comprising a segment of randomized sequence, the SELEX method includes steps of contacting the mixture with the target under conditions favorable for binding, partitioning unbound nucleic acids from those nucleic acids which have bound specifically to target molecules, dissociating the nucleic acid-target complexes, amplifying the nucleic acids dissociated from the nucleic acid-target complexes to yield a ligand-enriched mixture of nucleic acids, then reiterating the steps of binding, partitioning, dissociating and amplifying through as many cycles as desired to yield highly specific, high affinity nucleic acid ligands to the target molecule.
The basic SELEX method has been modified to achieve a number of specific objectives. For example, U.S. patent application Ser. No. 07/960,093, filed Oct. 14, 1992, entitled “Method for Selecting Nucleic Acids on the Basis of Structure,” describes the use of SELEX in conjunction with gel electrophoresis to select nucleic acid molecules with specific structural characteristics, such as bent DNA. U.S. patent application Ser. No. 08/123,935, filed Sep. 17, 1993, entitled “Photoselection of Nucleic Acid Ligands” describes a SELEX based method for selecting nucleic acid ligands containing photoreactive groups capable of binding and/or photocrosslinking to and/or photoinactivating a target molecule. U.S. patent application Ser. No. 08/134,028, filed Oct. 7, 1993, entitled “High-Affinity Nucleic Acid Ligands That Discriminate Between Theophylline and Caffeine,” describes a method for identifying highly specific nucleic acid ligands able to discriminate between closely related molecules, termed Counter-SELEX. U.S. patent application Ser. No. 08/143,564, filed Oct. 25, 1993, entitled “Systematic Evolution of Ligands by EXponential Enrichment: Solution SELEX,” describes a SELEX-based method which achieves highly efficient partitioning between oligonucleotides having high and low affinity for a target molecule. U.S. patent application Ser. No. 07/964,624, filed Oct. 21, 1992, entitled “Methods of Producing Nucleic Acid Ligands Ligands to HIV-RT and HIV-1 Rev,” now U.S. Pat. No. 5,496,938 describes methods for obtaining improved nucleic acid ligands after SELEX has been performed. U.S. patent application Ser. No. 08/400,440, filed Mar. 8, 1995, entitled “Systematic Evolution of Ligands by EXponential Enrichment: Chemi-SELEX,” now U.S. Pat. No. 5,705,337 describes methods for covalently linking a ligand to its target.
The SELEX method encompasses the identification of high-affinity nucleic acid ligands containing modified nucleotides conferring improved characteristics on the ligand, such as improved in vivo stability or improved delivery characteristics. Examples of such modifications include chemical substitutions at the ribose and/or phosphate and/or base positions. SELEX-identified nucleic acid ligands containing modified nucleotides are described in U.S. patent application Ser. No. 08/117,991, filed Sep. 8, 1993, entitled “High Affinity Nucleic Acid Ligands Containing Modified Nucleotides,” that describes oligonucleotides containing nucleotide derivatives chemically modified at the 5- and 2′-positions of pyrimidines. U.S. patent application Ser. No. 08/134,028, supra, describes highly specific nucleic acid ligands containing one or more nucleotides modified with 2′-amino (2′-NH2), 2′-fluoro (2′-F), and/or 2′-O-methyl (2′-OMe). U.S. patent application Ser. No. 08/264,029, filed Jun. 22, 1994, entitled “Novel Method of Preparation of 2′ Modified Pyrimidine Intramolecular Nucleophilic Displacement,” describes novel methods for making 2′-modified nucleosides.
The SELEX method encompasses combining selected oligonucleotides with other selected oligonucleotides as described in U.S. patent application Ser. No. 08/284,063, filed Aug. 2, 1994, entitled “Systematic Evolution of Ligands by Exponential Enrichment: Chimeric SELEX,” now U.S. Pat. No. 5,637,459. The SELEX method also includes combining the selected nucleic acid ligands with non-oligonucleotide functional units and U.S. patent application Ser. No. 08/234,997, filed Apr. 28, 1994, entitled “Systematic Evolution of Ligands by Exponential Enrichment: Blended SELEX”, now U.S. Pat. No. 5,683,867, and U.S. patent application Ser. No. 08/434,465, filed May 4, 1995, entitled “Nucleic Acid Ligand Complexes”, now U.S. Pat. No. 6,011,020. These applications allow the combination of the broad array of shapes and other properties, and the efficient amplification and replication properties, of oligonucleotides with the desirable properties of other molecules. Each of the above described patent applications which describe modifications of the basic SELEX procedure are specifically incorporated by reference herein in their entirety.
The present invention applies the SELEX methodology to obtain nucleic acid ligands to lectin targets. Lectin targets, or lectins, include all the non-enzymatic carbohydrate-binding proteins of non-immune origin, which include, but are not limited to, those described above.
Specifically, high affinity nucleic acid ligands to wheat germ agglutinin, and various selectin proteins have been isolated. For the purposes of the invention the terms wheat germ agglutinin, wheat germ lectin and WGA are used interchangeably. Wheat germ agglutinin (WGA) is widely used for isolation, purification and structural studies of glyco-conjugates. As outlined above, the selectins are important anti-inflammatory targets. Antagonists to the selectins modulate extravasion of leukocytes at sites of inflammation and thereby reduce neutrophil caused host tissue damage. Using the SELEX technology, high affinity antagonists of L-selectin, E-selectin and P-selectin mediated adhesion are isolated.